When operating a multi-phase step motor, the phase current to the motor must be accurately controlled to cause motor movement as well as to hold the motor position when stopped. The degree of control impacts stability while in motion and positional accuracy when stationary. One such method for controlling the operation of a stepper motor is described within U.S. Pat. No. 5,264,770 entitled “Stepper Motor Driver Circuit”. Other methods for controlling multi-phase step motors are found within U.S. Pat. No. 5,650,705 entitled “Apparatus and Method for Controlling Currents in an Inductor” and U.S. Pat. No. 6,049,184 entitled “Method and Arrangement for Controlling a Current”.
Such methods often utilize a fixed frequency pulse width modulator “PWM” consisting of a single comparator for each phase to determine when the phase current is at or above a set point. However, when the motor is moving at high speed the condition of excess current caused by back electromotive force, “EMF”, is difficult to determine such that the “PWM” frequency may not provide sufficient opportunity to control the motor current and there is no provision to stop the motor current based on a time factor.
One purpose of the present invention is to control the motor current independently in each motor phase of a multi-phase step motor to a set current value.